Typical operation in case concrete pipes are laid and buried underground is to dig the ground and to lay concrete pipes one after another into a ditch and to bury them.
For each linear section, the ground is dug down to the depth where concrete pipes are to be buried, and the concrete pipes are laid on a temporary base installed at the bottom of the ditch. In this case, adjustment is made in such manner that a direction and an inclination of the concrete pipes, in particular the lowermost position of inner side of each of the concrete pipes, are consistent with each other. Then, the concrete pipes are reburied.
These concrete pipes are used as passages for city water, sewage water or for passing other liquid and are laid with a certain inclination and without bending. If the buried concrete pipes are twisted and turned upward, downward, leftward or rightward, the liquid may stagnate or stop or leak into the soil, and the function as the passage cannot be fulfilled. Accordingly, an adequate reference line is required when the concrete pipes are to be laid.
A laser beam is suitable as such a reference line. The laser beam has no problem of slackening as in the case of threads even when used for a long distance, or the laser beam causes no hindrance to working. It is not cut off due to interference with operators or concrete pipes.
To form a reference line by a laser beam for the purpose of laying concrete pipes, a laser reference level setting device is used.
The laser reference level setting device comprises a laser sighting device and a target. The laser sighting device is placed at a first manhole or at starting point of a ditch, and the target is positioned at the other end of the ditch. The target has a marking at the center to indicate the center position, and the target is placed in such manner that the center position of the target coincides with the center position of the concrete pipe to be laid. The center position of the target is determined by a measuring instrument such as a theodolite, etc.
Next, A laser beam is emitted from the laser sighting device, and the laser sighting device is adjusted so that the laser beam is irradiated toward the center of the target. When the adjustment is completed, the laser beam emitted from the laser sighting device serves as the reference line for laying the concrete pipes.
However, a vibration occurs during construction such as a vibration caused by construction work or by vehicles passing by, and a displacement often occurs due to the vibration. Therefore, re-adjustment must be made when the construction is stopped and so positions of the laser sighting device and the target must be adjusted rather frequently.
Then, the laser reference level setting device and the target are proposed to detect the center of the target.
The laser reference level setting device comprises a laser sighting device and a target. Further, the laser sighting device comprises a laser oscillating device rotatably supported and having an emitting means for emitting a polarized laser beam and a light receiving means for receiving a laser beam reflected from the target, a driving unit for rotating the laser oscillating device in upward, downward, leftward or rightward directions and a control unit for driving the driving unit depending upon a light receiving condition of the laser beam reflected from the target. A reflection surface of the target is designed as a retroreflection surface having a reflection layer comprising small spheres or small prisms. On its left or right half, a 1/4 .lambda. birefringence member is attached.
When the target is scanned with a polarized laser beam coming from the laser sighting device to traverse and scan it in a horizontal direction and the laser beam is irradiated on the target, a polarized light beam is reflected, which is different from the polarized light beam irradiated from the reflection surface where the 1/4 .lambda. birefringence member is attached. On the other reflection surface, the light beam is reflected with the initial polarized light unchanged. The laser oscillating device can detect the center of the target by detecting the condition of the reflection light from the target, i.e. by detecting a point where the polarization condition changes. The control unit controls the driving unit and directs the laser beam toward the center of the target, and an irradiating position in a horizontal direction is determined. To determine the irradiating position in a vertical direction, the system is usually operated manually or by a radio control while watching the target and is adjusted toward the center.
After concrete pipes have been laid and buried, it should be confirmed that the concrete pipes have been laid as the predetermined setting and that there is no change over time in the buried condition of the concrete pipes when a certain time elapsed after burying. This is performed by measuring tilting of flow passage or tunnel. Description is now given on a conventional mode of measurement of tilting of a tunnel referring to FIG. 22.
In a tunnel 1, a concrete pipe 2 is laid, and manholes 3 and 4 are dug on an upstream end and a downstream end of the tunnel 1. At a predetermined position on the upstream end of the concrete pipe 2, e.g. at the lowermost position of the concrete pipe, a leveling instrument 5 is arranged and adjusted at a horizontal position and a staff 6 is erected at a vertical position based on the adjusted horizontal position of the leveling instrument 5. At a position on the downstream end of the concrete pipe 2 and matching the horizontally adjusted position, i.e. at the lowermost position, of the leveling instrument 5, a leveling instrument 7 is installed and adjusted at a horizontal position, and a staff 8 is erected in a vertical position based on the leveling instrument 7. A leveling instrument 9 is installed on the ground surface, and numeral values relating to the staffs 6 and 8 are read by the leveling instrument 9. From the difference between these values and from the distance between the staffs 6 and 8, a gradient of the tunnel 1 is determined.
To determine the irradiating position of the laser beam in a conventional type laser reference level setting device and the target, a 1/4 .lambda. birefringence member is attached on one-half of the reflection surface of the target, and the center of the target is detected according to the light receiving condition of the laser beam reflected from the target. Then, the position is automatically adjusted in such manner that the laser beam is irradiated toward the center. Therefore, on the reflection surface where the 1/4 .lambda. birefringence member is attached, a polarized laser beam is reflected, which has a polarizing condition different from that of the irradiated laser beam. On the other hand, the other reflection surface is an ordinary reflection surface where the irradiated laser beam is reflected while maintaining the initial polarizing condition. Accordingly, if a reflection surface which consists of metal, etc. is present at a position closer to the target plate, the reflection light maintaining the polarizing condition is reflected. As a result, the center of the target cannot be detected.
In many systems or instruments, positioning is carried out automatically in a horizontal direction but manually in a vertical direction. In case the buried pipe is extensively deviated, the laser beam is kept away from the target, and it is difficult to adjust. For this reason, operators must be assigned on both sides, i.e. on the laser reference level setting device side and the target side, and two or more operators are needed for positioning. This means lower working efficiency and much waste of time. If a mechanism to rotate by 90.degree. is installed on the target for automating the positioning operation in a vertical direction to save manpower, the system itself becomes more complicated, thus resulting in higher manufacturing cost.
In the conventional measurement of tilting of tunnel as described above, operators must enter the manholes 3 and 4 to install the leveling instruments 5 and 7 and to erect the staffs 6 and 8 in a vertical direction. For this purpose, additional operators must be assigned, and it is also difficult to perform accurate measurement.
To solve the above problems, it is an object of the present invention to provide a device, by which it is possible to detect the center of a target in reliable manner with simple arrangement, to adjust irradiating position of laser beam in vertical and horizontal directions with simple arrangement and without additional manpower, to assign only one operator for the purpose to save labor and to eliminate waste of time, to measure tilt angle of a tunnel after completion or of a tunnel already existing by only one operator, and to increase the measuring accuracy.